Extensive development has been carried out to apply the magnetoresistance effect, which is based on the conduction phenomenon depending on the spin of electrons, to a magnetic head and a magnetic random access memory (MRAM) or the like. The magnetoresistance effect is a phenomenon that a film stack including the structure of a ferromagnetic layer, a nonmagnetic layer and another ferromagnetic layer exhibits a change in the resistance depending on the relative angle between the magnetization directions of the ferromagnetic layers, which are opposed to each other across the nonmagnetic layer. In general, the minimum resistance is obtained when the magnetization directions are parallel, and the maximum resistance is obtained when the magnetization directions are anti-parallel.
Among film stacks exhibiting such magnetoresistance effect (magnetoresistance film stack), those in which a conductive material such as Cu is used for the nonmagnetic layer is generally called a giant magnetoresistance (GMR) film stack. One sort of GMR film stack is designed to flow a current in plane of the film faces, which is referred to as CIP-GMR film stack (Current In Plain GMR film stack), and another sort of GMR film stack is designed to flow a current perpendicularly to the film faces, which is referred to as CPP-GMR film stack (Current Perpendicular to Plain GMR film stack) On the other hand, a magnetoresistance film stack in which an insulating material such as alumina is used as the nonmagnetic layer is called a tunneling magnetoresistance (TMR) film stack.
The nonmagnetic layer formed of alumina or the like is referred to as the tunnel barrier layer. In the TMR film stack, a larger magnetoresistance change ratio (MR ratio) is obtained as the increase of the spin polarizability in the magnetic layers, which are opposed across the nonmagnetic layer. Materials having a large spin polarizability include magnetic metals such as Fe, Co and Ni, magnetic metal alloys such as a Co—Fe alloy and a Ni—Fe alloy, and half metallic ferromagnetic materials, which are expected to achieve a spin polarization of 100%. The TMR film stack described above is expected as a magnetoresistance film stack to exhibit a larger MR ratio.
A magnetoresistance film stack classified as a spin valve type has been proposed in order to apply the magnetoresistance film stack such as the GMR and TMR film stack to devices operating on a micro-magnetic field. In a spin valve type magnetoresistance film stack, one of magnetic layers opposed across a nonmagnetic layer has a fixed magnetization direction, and the other has a magnetization direction freely reversible on an external magnetic field. The magnetic layer with the fixed magnetization is referred to as the fixed (ferro) magnetic layer (or the magnetization fixed layer), while the magnetic layer with the freely reversible magnetization is referred to as the free (ferro) magnetic layer (or the magnetization free layer). This architecture allows the relative angle between the magnetization directions of the fixed and free magnetic layers to be easily changed.
The fixed magnetic layer can be mainly realized by the following three approaches. A first approach is to use magnetic material having a coercive force larger than that of the free magnetic layer, so that the difference between the coercive forces of the free and fixed magnetic layers is used. A second approach is to laminate a magnetic layer that functions as the fixed magnetic layer with a permanent magnet film or the like having a large coercive force, so that the magnetization of the fixed layer is fixed by using a coupling magnetic field generated across the interface between these films. A third approach (an exchange coupling technique) is to laminate a magnetic film that functions as the fixed magnetic layer with an antiferromagnetic layer, so that magnetization of the fixed magnetic layer is fixed by an exchange coupling magnetic field generated across the interface between these films. The third approach has been already applied to magnetic heads in practical use with respect to the GMR film stack. On the basis of this actual achievement, the exchange-coupling-based approach is also expected to be applied to next-generation magnetic heads and MRAMs and so on, with respect to the spin valve type magnetoresistance film stack using the TMR film stack as well as that using the GMR film stack.
In the application to a magnetic head or an MRAM, the magnetoresistance film stack requires heat resistance against high temperature processes in the device manufacture. In the application to an MRAM, for example, the heat resistance against temperatures of 350° C. or higher is required. However, a heat treatment conducted at such a high temperature sometimes deteriorates the magnetoresistance characteristics and thereby decreases the MR ratio. It is considered that one cause of this deterioration is interdiffusion of elements of the film stack within the device.
FIG. 1 is a cross sectional view showing an example of the structure of the spin valve type TMR film adopting the exchange coupling as a magnetoresistance film. On a substrate 109 provided is a film structure of a Ta film of 30 nm as a lower electrode layer 106, an NiFe film of 3 nm as a foundation layer 105, a PtMn film of 15 nm as an antiferromagnetic layer 104, a three-layered film stack including a CoFe film of 3 nm, an Ru film of 0.8 nm and a CoFe film of 3 nm as a fixed ferromagnetic layer (magnetization fixed layer) 103, an aluminum oxide film of 1 nm as a nonmagnetic layer (tunnel barrier layer) 102, an NiFe film of 5 nm as a free ferromagnetic layer (magnetization free layer) 101, and a Ta film of 20 nm as an upper electrode layer 107, which are sequentially laminated from the side of the substrate 109 in this order.
FIG. 2 is a graph showing distribution of manganese in TMR film stacks subjected to heat treatment at 275° C. and 350° C., respectively. FIG. 2 shows the evaluation result using a secondary ion mass spectrometry (SIMS) technique. The manganese of the PtMn film, which functions as the antiferromagnetic layer 104, diffuses in accordance with the increase of the temperature in the heat treatment. In particular, accumulation of the manganese is observed in the vicinity of the nonmagnetic layer 102. It is probable that the accumulated manganese influences characteristics of the barrier and causes the decrease of the MR ratio. Accordingly, the suppression of the manganese diffusion is probably important to obtain the TMR film with high heat resistance.
An approach in which an iron oxide film (FeOx) is inserted between the nonmagnetic layer 102 and the fixed ferromagnetic layer 103 (CoFe) to suppress the manganese diffusion is disclosed in “40% tunneling magnetoresistance after anneal at 380° C. for tunnel junctions with iron & #8211; oxide interface layers”, Zongzhi Zhang, S. Cardoso, P. P. Freitas, X. Batlle, P. Wei, N. Barradas, and J. C. Soares, J. Appl. Phys., 89 (2001) p. 6665. This document described that the disclosed approach achieves 40% MR ratio after heat treatment at 380° C. However, this approach suffers from an extremely narrow margin of heat treatment conditions for obtaining a large MR ratio has an, which prevents the increase of yield in manufacturing. Moreover, magnetic characteristics of iron oxide are largely changed by heat treatment. Since the fixed ferromagnetic layer has a stacked structure of an iron oxide film and a CoFe film, the intensity of magnetization is changed by heat treatment in the fixed ferromagnetic layer as a whole. The magnetization of the fixed ferromagnetic layer has an effect on the free ferromagnetic layer when the TMR film is patterned. For example, in the case of the MRAM, the effect on the free ferromagnetic layer caused by the magnetization of the fixed ferromagnetic layer generates offset in magnetization reversal characteristics of the free ferromagnetic layer. This offset constantly changes in the stack structure of the iron oxide film, which make it difficult to have a stable device operation.
The use of a CoFe/CoFeOx/CoFe structure, in which a CoFeOx layer is inserted between CoFe layers of a fixed ferromagnetic layer, is also disclosed in “Improved Thermal Stability of Ferromagnetic Tunnel Junction With a CoFe/CoFeOx/CoFe Pinned Layer”, T. Ochiai, N. Tezuka, K. Inomata, S. Sugimoto, and Y. Saito, Journal of Magnetic Society of Japan, vol. 27, No. 4 (2003), p. 307. This document describes that the disclosed approach achieves a maximum MR ratio of 47% after heat treatment at 350° C. However, it is pointed out that the use of the CoFeOx layer suffers from oxygen diffusion after heat treatment even at a relatively low temperature. In other words, magnetic characteristics of the fixed ferromagnetic layer are changed at relatively low temperatures, making it difficult to achieve a stable device operation as is the case of the insertion of the FeOx layer described above.
As a related technique, Japanese Laid-Open Patent Application No. JP-A 2004-47583 discloses a magnetoresistance element, a magnetic memory, the magnetic head and a magnetic storage device. According to this conventional technique, the size of crystal grains of an antiferromagnetic layer and the thickness of a magnetization fixed layer are defined so as to suppress the effect of the manganese diffusion. That is, the size of crystal grains of an antiferromagnetic layer and the thickness of a magnetization fixed layer satisfies the following:1 nm≦D<10 nm, and D≧2×H or H≧1.4×D orD≧10 nmwhere D is the average grain diameter of crystal grains of material of the antiferromagnetic layer, and H is the distance between the antiferromagnetic layer and a nonmagnetic layer (barrier layer).
FIG. 3 is a cross sectional view showing the structure of the TMR film in this conventional technique. When crystal grain boundaries are consecutively formed in the antiferromagnetic layer 104 and the fixed magnetic layer 103, it is difficult to obtain a remarkable effect of the heat resistance improvement even in the case that D and H are defined as described above, since diffusion is enhanced by grain boundaries 110.
In this conventional technique, disclosed further is a method in which a diffusion control layer 112 is inserted between the antiferromagnetic layer 104 and the nonmagnetic layer 102. FIG. 4 is a cross sectional view showing the structure of the TMR film stack in accordance with this conventional technique. The diffusion control layer 112 here contains a ferromagnetic material having a composition represented by a formula of M-X, where M represents at least one element selected from Fe, Co and Ni, and X represents at least one nonmagnetic element selected from the IVa group to VIIa group, the VIII group, the Ib group, lanthanoids, Al, Si, Sc, Y, Zn, Ga, Ge, B, C, N, O, P and S. In this case, the grain boundaries 110 are formed as shown in FIG. 4, and therefore the diffusion through the grain boundaries 110 are suppressed in comparison with FIG. 3.
FIGS. 5 to 7 are cross sectional views of the TMR film showing states of the Mn diffusion in this conventional technique. In this case, for example, progress of the diffusion of the manganese of the antiferromagnetic layer is observed as shown in FIG. 5, and manganese 111 is accumulated between the layers. The diffusion is temporarily suppressed here. Thereafter, however, the accumulated manganese 111 reaches the grain boundaries 110 within the upper layer. As shown in FIG. 6, the diffusion is then progressed to further upper layers through the grain boundaries 110 of the upper layer. Finally, the diffusion reaches the nonmagnetic layer 102 as shown in FIG. 7. That is, a problem occurs in which the MR ratio is sharply decreased after heat treatment over a certain heat treatment temperature or a certain heat treatment time. This problem particularly depends on the ultimate pressure, the substrate temperature, the sputtering gas pressure, and the sputtering power, which are conditions of the deposition of the respective layers through sputtering. When the sputtering gas pressure is increased from 0.1 Pa to 1 Pa, for example, the MR ratio is decreased even at a low heat treatment temperature of about 300° C. When the substrate temperature is increased up to 150° C., heat resistance is decreased in comparison with the case of film formation at the room temperature. This implies that strict control of film formation conditions is required in order to prepare the TMR film with excellent heat resistance, causing the decrease of manufacturing yield and the increase of manufacturing cost.
In addition, Japanese Laid-Open Patent Application No. JP-A2004-47583 discloses a case in which ferromagnetic material functioning as a grain boundary control layer and having a compound indicated by a formula of M-X has an amorphous structure. More specifically, the use of CoFeB and NiFeB is disclosed. These amorphous structure films, however suffer from not only a problem that grain boundaries are formed by crystallization in heat treatment at a high temperature, but also a problem that the diffusion of boron is observed in heat treatment at 300° C. or lower; the barrier characteristics are deteriorated by the diffusion of boron, instead of the diffusion of manganese of the antiferromagnetic layer.
Japanese Laid-Open Patent Application No. JP-A Heisei 9-23031 discloses a magnetoresistance film stack using a soft magnetic film, characterized in that crystal grains of an element X with an average crystal grain diameter of 20 nm or less is separated from carbide or nitride of an element M within the soft magnetic film having a compound of X-M-Z, where the element X represents one or two or more of elements selected from Fe, Co and Ni, and the element M represents one or two or more of elements selected from Ti, Zr, Hf, V, Nb, Ta, Mo and W, while the element Z represents one or two of elements selected from C and N. When crystal grains of the element X are separated from carbide or nitride of the element M, however, crystal grain boundaries exist therebetween, and this causes a problem that the Mn diffusion progresses through the crystal grain boundaries, and the MR ratio is sharply decreased after heat treatment over a certain heat treatment temperature or a certain heat treatment time.
There is a need of a magnetoresistance element which incorporates a heat-resistive magnetoresistance film stack which does not exhibit deterioration of characteristics after a heat treatment process at a temperature of 350° C. or higher. There is a need of a magnetoresistance element with high manufacturing yield and low manufacturing cost. There is a need of a magnetic random access memory, the magnetic head and the magnetic storage device that exhibit a high heat resistance, a high manufacturing yield and the low manufacturing cost.
As a related technique, a ferromagnetic tunnel junction element and a manufacturing method thereof are disclosed in Japanese Laid-Open Patent Application No. JP-A 2002-158381. This ferromagnetic tunnel junction element includes: an antiferromagnetic layer containing manganese; a magnetization fixed layer formed on the anti ferromagnetic layer in which a pair of first and second ferromagnetic layers are opposed across an insulating layer or an amorphous magnetic layer; a tunnel barrier layer formed on the magnetization fixed layer, and a magnetization free layer formed on the tunnel barrier layer. The insulating layer or the amorphous magnetic layer in the magnetization fixed layer may have a function of avoiding the diffusion of manganese contained in the antiferromagnetic layer. The insulating layer in the magnetization fixed layer may be formed through exposing the first ferromagnetic layer of the magnetization fixing layer in an oxidizing atmosphere, a nitriding atmosphere and a carbiding atmosphere.
As a related technique, a magnetoresistance type magnetic head and a magnetic recording/reproducing device are disclosed in Japanese Laid-Open Patent Application No. JP-A 2002-150514. In this magnetoresistance type magnetic head, an antiferromagnetic film, a fixed layer, a nonmagnetic film and a free layer are sequentially deposited. The fixed layer is provided with a ferromagnetic film with the magnetization direction fixed to an external magnetic field by exchange coupling with the antiferromagnetic film. The free layer is provided a ferromagnetic film in which the magnetization is rotated in accordance with the external magnetic field. The magnetoresistance effect is observed due to the change in the relative angle between the magnetizations of the ferromagnetic films within the fixed and free layers. In the fixed layer, an oxide film is formed between a pair of the ferromagnetic films. The film thickness of the oxide film is 5×10−10 m or larger and 30×10−10 m or less. The oxide film contains at least one element selected from Mg, Al, Si, Ca, Ti and Zr.
As a related technique, a magnetoresistance element and a magnetoresistance type head are disclosed in Japanese Laid-Open Patent Application No. JP-A 2001-352112. The magnetoresistance element is comprised of a film stack of an antiferromagnetic layer, a magnetic layer (fixed layer), a nonmagnetic layer, and a magnetic layer (free layer), which are sequentially laminated. The magnetization of the fixed layer is not easily rotated by the external magnetic field. The magnetization of the free layer is easily rotated by the external magnetic field. An oxide layer is formed within one of the antiferromagnetic layer, the fixed layer, the nonmagnetic layer and the free layer, or on the face of one of the antiferromagnetic layer, the fixed layer, the nonmagnetic layer and the free layer. In addition, an oxygen diffusion preventing layer is formed between the oxide layer and at least one of the layers (referred to as the other layer hereinafter) selected from the antiferromagnetic layer, the fixed layer, the nonmagnetic layer and the free layer in order to suppress oxidization of the said other layer. The oxygen diffusion preventing layer may mainly consist of at least one selected from Au, Pt, Ag, Ru, Ni and an alloy of Ni1-xMx (where M represents more than one kind among Fe, Co, Cr and Ta, 0≦X≦40, and X represents the atomic composition ratio). The oxide layer may be composed of oxide mainly consisting of an element D (D represents at least one kind of element selected from Al, Si, Ti, Ta, Fe, Co and Ni).
Japanese Laid-Open Patent Application No. JP-A 2001-236607 discloses a technique of a thin film magnetic head and a manufacturing method thereof. This thin film magnetic head includes a recording head. The recording head is formed by a read head, a lower magnetic core layer, an upper magnetic core layer, and a coil layer. The composition of the two magnetic core layers are expressed by a composition formula of FeaMgbNbcOd (subscripts a, b, c and d indicate atomic weight %). The composition is characterized by being composed of a soft magnetic material in a range of a+b+c+d=100, 45≦a≦85, 5.5≦b≦28, 0.5≦c≦3 and 8≦d≦35. Metal magnetic crystal grains with an average crystal grain diameter of 15 nm or less may be formed essentially by Fe, while the soft magnetic material forming a grain boundary product for substantially covering the metal magnetic crystal grains may be composed by Mg, Nb and O.
As a related technique, a magnetoresistance film and a magnetic read sensor using thereof are disclosed in Japanese Laid-Open Patent Application No. JP-A 2001-6932. This magnetoresistance film includes a regular antiferromagnetic layer, a fixed magnetic structure portion coupled thereto, a nonmagnetic conductive layer, and a free magnetic layer portion having at least one magnetic layer. The fixed magnetic structure portion includes a multilayer film structure including one or more pairs of a three-layer structure made of a first ferromagnetic layer, a nonmagnetic intermediate layer, and a second ferromagnetic layer, in which the first and second ferromagnetic layers are parallel or have a parallel component with each other in the magnetization direction.